ear to the ground, to have/keep an

keep an ear to the ground

 and have an ear to the ground; keep one's ear to the ground; have one's ear to the ground
Fig. to devote attention to watching or listening for clues as to what is going to happen. John had his ear to the ground, hoping to find out about new ideas in computers. His boss told him to keep his ear to the ground so that he'd be the first to know of a new idea.
See also: ear, ground, keep

have (or keep) an ear to the ground

be well informed about events and trends.
The idea behind this phrase is that by putting your ear against the ground you would be able to hear approaching footsteps.
See also: ear, ground, have

ear to the ground, to have/keep an

To be well informed. The allusion here, one writer conjectures, is to the days of cowboys and Indians, when one literally put one’s ear to the ground in order to hear the sound of horses miles away. An Americanism dating from the late nineteenth century, the term was a cliché by the time Stanley Walker poked fun at it (and two others) in The Uncanny Knacks of Mr. Doherty (1941): “He had his ear to the ground and his eye on the ball while they were sitting on the fence.”
See also: ear, have, keep